Meaning
A gas displacement technique measures the true volume of solid objects including porous or irregularly shaped polymer samples. Utilizing helium pycnometry allows for the calculation of skeletal density by excluding the volume of open pores and surface voids. It is preferred over liquid displacement for materials where the immersion fluid might be absorbed or fail to wet the surface.
Skeletal Volume
The small size of the helium atom allows the gas to penetrate even the smallest cracks in a moulded part. Performing helium pycnometry ensures that the measured volume is the actual space occupied by the polymer molecules. This precision is necessary when calculating the density of foams or sintered parts where internal air pockets are common.
Gas Permeation
The use of an inert gas prevents chemical reactions with the specimen during the test. Unlike water, the gas used in helium pycnometry does not have surface tension issues that lead to trapped air bubbles. This results in a more accurate volume measurement than hydrostatic weighing for complex geometries.
Porosity Exclusion
Comparing the skeletal density to the bulk density reveals the total void space within a component. Helium pycnometry provides a non-destructive way to verify the integrity of a part after a foaming process. This data helps engineers adjust the injection pressure or cooling time to eliminate unwanted internal voids.
The technique is a standard for verifying the density of high performance engineering resins where air entrapment would compromise structural safety.
